RemoveDebris is an EU (European
Union) Framework 7 (FP7) research project to develop and fly a low cost
in-orbit demonstrator mission that aims to de-risk and verify
technologies needed for future ADR (Active Debris Removal)
missions. It is not an end-to-end demonstration of a full ADR mission.
However, it will demonstrate the use, on-orbit, of some of the key
aspects of a ‘real’ ADR mission. 1)2)

RemoveDebris is aimed at performing
key ADR technology demonstrations (e.g., capture, deorbiting)
representative of an operational scenario during a low-cost mission
using novel key technologies for ADR. The project is based on and aimed
at contributing to global/European ADR roadmaps.

A microsatellite called here
RemoveSAT, will release, capture and deorbit two space debris targets,
called DebrisSats, in sequence using various rendezvous, capture and
deorbiting technologies thus demonstrating in orbit, key ADR
technologies for future missions in what promises to be the first ADR
technology mission internationally. 3)4)5)6)7)8)

The debris objects themselves in
this case will be released by the main satellite with subsequent
recapture. Although this is not a fully-edged ADR mission, the project
is an important step towards a fully operational ADR mission. The
ultimate goal of this activity is to protect space assets from space
debris and to minimize the collision risk of current and future space
missions as the FP7 call for space calls for.

Stellenbosch University – Electronic System Laboratory (ESL), South Africa

CubeSat ADCS hardware and software

Table 1: Overview of the project team

Some background:

One of the most active in the field
of debris removal is ESA (European Space Agency). ESA has produced a
range of CleanSpace roadmaps, two of which focus on (a) space debris
mitigation and (b) technologies for space debris remediation. A main
part of these roadmaps is e.Deorbit, a program spanning a host of phase studies examining removing a large ESA-owned object from space, namely Envisat. 9)10)

This initiative started with ESA's
SOADR (Service Orientated ADR) Phase 0 study involving the analysis of
a mission that could remove very heavy debris from orbit examining both
the technical challenges and the business aspects of multiple ADR
missions. Progressing on, ESA has also now completed Phase A and Phase
B1 studies, with several more mature designs now available. ESA's SSBB
(Satellite Servicing Building Blocks) study originally examined remote
maintenance of geostationary telecommunications satellites using a
robotic arm. The French space agency, CNES, is also widely involved in
debris removal and has funded studies such as OTV which traded-off
different ADR mission scenarios.

Regarding
the development of capture technologies, there are several on-going
efforts. Airbus DS capture designs include the robotic arm, net, and
harpoon demonstrators for use in space. The First European System for
Active Debris Removal with Nets (ADR1EN) is testing net technologies on
the ground with the aim of commercializing later on. A host of other
capture technologies have also been proposed including: ion-beam
shepherd, gecko adhesives and polyurethane foam. Aviospace have been
involved with some ADR studies such as the CADET (Capture and
De-orbiting Technologies) study which is examining attitude estimation
and non-cooperative approach using a visual and infrared system and the
HADR (Heavy Active Debris Removal) study that examined trade-offs for
different ADR technologies, especially including flexible link capture
systems.

Although recently there have been
advances in relative space navigation, the complex application of fully
uncooperative rendezvous for debris removal has not yet been attempted.
VBN (Vision-Based relative Navigation) systems, which would be
necessary for future debris removal missions are currently being
developed and will be demonstrated on RemoveDebris. Other recent
research specifically related to VBN for debris removal includes: TU
Dresden, Thales, and Jena-Optronik.

Regarding rendezvous in space, the
ATV (Autonomous Transfer Vehicle) of JAXA was one of the first times a
spacecraft initiated and commenced a docking maneuver in space in a
fully autonomous mode. The ETS-VII (Engineering Test Satellite VII) ,
alias KIKU-7, by JAXA in 1997 was one of the first missions to
demonstrate robotic rendezvous using chaser and target satellites . The
ADRV (AoLong-1 `Roaming Dragon' Vehicle) was also recently launched by
CNSA (China National Space Administration) in 2016 in order to test
target capture with a robotic arm; results are presently not available.
Most recently JAXA's HTV-6 vehicle, which launched in early 2017,
unsuccessfully attempted to deploy an electrodynamic tether under the
KITE (Kounotori Integrated Tether Experiment. 11)

Upcoming missions to tackle debris
removal include CleanSpace One by EPFL (Ecole Polytechnique Federale de
Lausanne), Switzerland, which aims to use microsatellites with a
grabber to demonstrate capture. 12)
The mission is still under design and launch is not foreseen for a few
years. As mentioned previously, ESA's e.Deorbit will likely result in a
large scale mission and is currently proposed for 2023. Of interest is
AstroScale, a company based in Singapore, aiming to launch a mission
with thousands of `impact sensors' to build up knowledge of the
magnitude of small fragments.

• The mission has as its
primary aim, the raising of Technology Readiness Levels (TRL), and
gaining on-orbit experience with:

• The core concept behind
the mission, is to use a small-satellite (~100kg) as a
‘mothership’, on which the payloads are carried, and from
which CubeSats (~3 kg) are released and used as
‘pseudo-debris’ targets

• The “RemoveDebris” mission obviously does not want to produce any excess debris in orbit from its activities.

• The current mission
baseline is therefore to fly the mission at low altitude (<400km)
where orbital residence times for the different parts of the mission
will be low (<3 years)

• Parts of the mission
involving close proximity operations between the mothership and
CubeSats have been designed to be passively safe (objects will
naturally drift apart).

• The consortium is in discussions with several launch providers and no launch has been yet selected.

• One potential option is the US company NanoRacks to obtain a launch from the International Space Station (ISS).

Table 2: RemoveDebris mission overview

In-orbit demonstrations:

1) Net experiment: The net
scenario is shown in Figure 1 and is designed to help mature net
capture technology in space. In this experiment, initially the first
CubeSat (net), DSAT-1 (DebrisSat-1), is ejected by the platform at a
low velocity (~ 0.05 m/s). DSAT-1 proceeds to inflate a balloon which,
as well as acting as a deorbiting technology, provides a larger target
area. A net from the platform is then ejected at the balloon. Once the
net hits the target, deployment masses at the end of the net wrap
around and entangle the target and motor driven winches reel in the
neck of the net preventing re-opening of the net. The CubeSat is then
left to deorbit at an accelerated rate due to the large surface area of
the balloon.

2) Harpoon experiment: The harpoon scenario is shown in Figure 2.
In this experiment, the second CubeSat (harpoon), DSAT-2, is ejected by
the platform at a low velocity. Shortly after ejection, the CubeSat
releases target panels to increase the target surface area. The
platform GNC system aligns the harpoon with the center of the CubeSat
then fires the harpoon. The harpoon is designed with a flip-out locking
mechanism that prevents the tether from pulling out of the CubeSat. The
CubeSat is then left to deorbit.

3) VBN experiment: The VBN
experiment is shown in Figure 3. In this experiment, the third CubeSat
(harpoon), DSAT-3, is ejected by the platform. The VBN system
(including LiDAR) uses the previous net and harpoon experiments to
calibrate itself. A series of maneuvers are then undertaken allowing
the VBN system and supervision cameras to collect data and imagery
which are later post-processed on ground.

Other Experiments: The
RemoveDebris mission, in addition to performing the three primary
experiments, also aims to test a few other developed technologies.
These include the EP (Electric Propulsion) system and a 10 m2
dragsail. These experiments are to be performed after the primary
experiments and are explained in further depth in the payload section.

RemoveSat platform

The RemoveDebris platform,
RemoveSat, is a derivative of SSTL’s next generation small
satellite platforms, the SSTL X50 series. The RemoveDebris mission
provides an excellent opportunity to demonstrate the capabilities of
this new platform. The X50 series platform is built on 30 years of
experience and success in SSTL of breaking low cost barriers and
delivering operational level performance in small satellite packages.
The X50 architecture is based on a modular and expandable philosophy
that utilizes common modules. This allows the system to be adaptable to
varying mission applications and requirements.

Mass

~100 kg

Envelope

0:65 m x 0:65 m x 0:72 m

Downlink, uplink

2 Mbit/s (S-band), 19.2 kbit/s

Payload mass, payload power

~40 kg, 8 W OAP (high peaks possible)

Pointing knowledge

~2.5º

ΔV

Up to 6 m/s

Table 3: Key parameters of the RemoveDebris platform

Figure 4: RemoveSat platform configuration (image credit: SSTL)

The platform, shown in Figure 4 and Figure 5,
is based on four side panels, a payload panel, and a separation panel.
The payloads are mounted either on the payload panel within the payload
volume atop the avionics bay or along the side panels as required. The
side and payload panels are made from aluminum honeycomb sandwich
panels while the separation panel is made out of machined aluminum.
Three of the four side panels are also populated with solar cells to
provide power throughout the orbit. Below the payload panel is the
platform avionics bay where the platform subsystems are housed. This
includes items such as magnetometers, magnetorquers, reaction wheels,
gyros, on-board computers, GPS receiver, X50 avionics stack, and
batteries.

The X50 avionics system builds on
the modular and expandable philosophy and also improves
manufacturability, integration, and testing. The avionics system is
based on a card-frame structure with backplane interconnections. This
results in far less labor to interconnect the modules and also
simplifies integration and module insertion and replacement. The new
modules that have been developed for the X50 avionics include: PDM
(Power Distribution Module), BCM (Battery Charge Module), STRx (S-band
Transmitter/Receiver), PIU (Payload Interface Unit), CAN bridge, ADCS
and data handling elements all accommodated in card format within a
single card frame assembly. Redundancy of primary systems (transceiver,
Power BC, OBC) is achieved through simple duplication of the relevant
cards. The card frame assembly is completed by means of a card frame
which provides interfaces and connections between all of the core
avionics elements, whilst also reducing platform harness mass and
complexity.

Figure 6: Card frame assembly of the platform (image credit: SSTL)

The remainder of the platform is
made up of heritage SSTL subsystems and equipments. A full equipment
list for the platform is included in Table 4.

The RemoveDebris platform design
accommodates the majority of the platform avionics and equipments in
the lower half of the spacecraft, with the upper half of the spacecraft
primarily dedicated to accommodating the RemoveDebris payloads. The
spacecraft configuration can be seen in Figure 7.

Accommodation study: The
RemoveDebris platform design accommodates the majority of the platform
avionics and equipments in the lower half of the spacecraft, with the
upper half of the spacecraft primarily dedicated to accommodating the
RemoveDebris payloads. The upper bay of the platform accommodates all
of the RemoveDebris payloads, in line with the mission profile and
operations concept which essentially requires all payloads to be
deployed in the same direction (and monitored in that direction).

Design principles and drivers: The
X-Series platforms are being developed with some key drivers and
principles in mind. These are a combination of (a) principles that SSTL
have employed successfully in delivering small satellites in the last
25 years, and (b) new approaches that are enabled by SSTL’s
evolution as a company in the last 10 years, specifically the recently
developed in house capabilities for batch/mass production and automated
test. These key drivers and principles can be summarized as follows:

• The use of mature, well
developed non space specific protocols such as CAN (Controller Area
Network)and LIN (Local Interconnect Network).

• On board autonomy, resulting in the elimination of the need for expensive, constantly manned ground segments.

• Robustness and redundancy;
simple and robust operational modes that deliver competitive payload
availability performance with multiple backup functionality and
equipments on board to assure mission lifetime and guard against
unforeseen and random outages and failures.

• The use of COTS
(Commercial-off-the-Shelf) components and technologies building on over
25 years of successful implementation on operational missions.

• Modularity; investing the
development of only a few key new systems that can be arranged in
configurations to deliver a wide variety of performance and capacity
variations depending on mission requirements.

The main RemoveSat spacecraft uses the X50M variant structure, shown in Figure 7,
and is based on the X50 platform and avionics, with modifications to
meet the mission specific requirements. The core avionics comprises the
systems required to support the payloads, including power system,
communications, and data handling.

Figure 8
shows the system block diagram for RemoveDebris and shows the different
subsystem elements and their interactions. The system block diagram
uses the typical subsystem division used by SSTL. The different
subsystems defined are:

There is a CAN (Controller Area
Network) bus that is distributed across the spacecraft and is used to
transmit and receive data from other CAN connected elements. In
addition to the CAN bus, the X50 avionics introduces a dual redundant
LIN (Local Interconnect Network) bus used to communicate between the
power system modules and distributed switches. The LIN master on the
STRx card is used to bridge the telemetry and telecommands between the
LIN and CAN buses.

Payload and housekeeping data will
be stored on board the satellite and will be transmitted to the ground
via its S-band downlink antenna to the ground stations based in
Guildford, UK. A larger ground station is also available either as a
backup or as the main ground station in Bordon, UK. The 2-4 Mbit/s
downlink rate is readily supported by the X50 S-band transceiver and
the heritage ground station equipment. Telecommand and Telemetry uplink
and downlink are via a low rate S-band system. LEOP and emergency
communications are also performed using the low rate S-band system.

Payload Panel Layout: Most of the payloads are attached to the side panels above the payload panel (Figure 9).
The net payload is attached to the payload panel via a cylindrical
tube. The net is placed as close as possible along the CoG axis in
order to reduce any rotation torques as it is deployed. The locations
of the CubeSat deployers are placed in close proximity to the other
payloads required for their respective demonstrations. For example, the
CubeSat dedicated for the net demonstration is close to the net
payload, while the VBN dedicated CubeSat is close the VBN sensor and
ISL. The location and angle of attachment of the supervision cameras
are optimized to provide the best possible view of all of the
demonstrations.

Platform Internal Configuration and AIT (Assembly, Integration and Test): Figure 10
shows the platform in an open configuration; this is how the platform
will be built up in stages during the AIT phase. The first thing that
gets brought to the integration table is the separation panel. The side
panels then get attached, via hinges, to the separation panel and are
supported by the integration table in an open “flower
petal” configuration. Payload panel support rods (the red colored
rods shown in Figure 10) are then
attached to the separation panel. This allows the payload panel to be
attached on top. With all the main structural panels in place, the
subsystems and payloads can be integrated one-by-one to the spacecraft.
In this configuration, the platform can be slowly soft-stacked allowing
for fit checks, interface tests, and system end-to-end tests to be
completed.

For the
final hard-stack, all payload and subsystem bolt fixings to the
structure are torqued to appropriate levels and head-staked. The side
panels are then folded up and bolted to the payload panel one-by-one.
The hinge joints and the payload panel support rods are removed from
the structure. The final structural bolts are torqued and head-staked
and the platform is ready for integration into the launch container
ready for transport.

• December 18, 2017: SSTL has
shipped the RemoveDebris spacecraft to the Kennedy Space Center in
Florida for launch to the ISS (International Space Station) inside a
Dragon capsule on board the SpaceX CRS-14 resupply mission, a service
provided through supply agent, NanoRacks. 13)

Launch: The RemoveDebris
microsatellite was launched on 2 April 2018 (20:30 UTC) with the
Falcon-9/Dragon vehicle of SpaceX CRS-14 (cargo delivery). The launch
site was Cape Canaveral Air Force Station, FL. 14)15) The flight is being conducted under the Commercial Resupply Services contract with NASA. 16)

This flight delivers scientific
investigations looking at severe thunderstorms on Earth, the effects of
microgravity on production of high-performance products from metal
powders, and growing food in space. Dragon also carries cargo for
research in the National Laboratory, operated by CASIS (Center for the
Advancement of Science in Space), including testing the effects of the
harsh space environment on materials, coatings and components;
identifying potential pathogens aboard the station; and investigating
an antibiotic-releasing wound patch.

Dragon is packed with 2625 kg of research, crew supplies and hardware to be delivered to the station:

ASIM (Atmosphere-Space
Interactions Monitor), an ESA science instrument (314 kg) to be
installed on the Columbus External Platform Facility (CEPF). ASIM
surveys severe thunderstorms in Earth’s atmosphere and
upper-atmospheric lightning, or transient luminous events. These
include sprites, flashes caused by electrical break-down in the
mesosphere; the blue jet, a discharge from cloud tops upward into the
stratosphere; and ELVES, concentric rings of emissions caused by an
electromagnetic pulse in the ionosphere.

Wound Healing. NanoRacks
Module 74 Wound Healing tests a patch containing an antimicrobial
hydrogel that promotes healing of a wound while acting as a scaffold
for regenerating tissue. Reduced fluid motion in microgravity allows
more precise analysis of the hydrogel behavior and controlled release
of the antibiotic from the patch.

The Canadian Space Agency’s
study Bone Marrow Adipose Reaction: Red or White (MARROW) will look at
the effects of microgravity on bone marrow and the blood cells it
produces – an effect likened to that of long-term bed rest on
Earth. The extent of this effect, and bone marrow’s ability to
recover when back on Earth, are of interest to space researchers and
healthcare providers alike.

Understanding how plants respond to
microgravity also is important for future long-duration space missions
and the crews that will need to grow their own food. The PONDS (Passive Orbital Nutrient Delivery System) arriving on Dragon uses a newly-developed passive nutrient delivery system and the Veggie
plant growth facility currently aboard the space station to cultivate
leafy greens. These greens will be harvested and eaten by the crew,
with samples also being returned to Earth for analysis.

Overview-1A, a 3U CubeSat
(4.2 kg) of SpaceVR (Space Virtual Reality), a crowd-funded mission
based on a Pumpkin platform, USA. The goal is to allow users to
‘experience space firsthand’ using any mobile, desktop, or
virtual reality device.

1KUNS-PF (1st Kenyan
University NanoSatellite-Precursor Flight), a 1U CubeSat developed at
the University of Nairobi, Kenya in collaboration with “La
Sapienza” University of Rome and ASI (Italian Space Agency). A
technology demonstration mission.

Orbit: Near-circular orbit, altitude = 400 km, inclination = 51.6º.

Originally, the RemoveDebris mission
was intended to be flown on a traditional launcher into a higher SSO
(Sun-Synchronous Orbit). However, the mission baseline has been revised
to take into account feedback from international and national space
policy providers in terms of risk and compliance and a potential launch
option has been found: deployment from the ISS (International Space
Station). Work is on-going with NanoRacks to define the details (Ref. 5).

To be deployed from the ISS, the
satellite must first get there. This is done by loading the
RemoveDebris platform onto a resupply mission to the ISS. The
RemoveDebris satellite will be shipped in a launch box. The launch box
serves two main purposes:

- Protect astronauts from protrusions and sharp edges during handling aboard the ISS.

The launch box will be wrapped in
foam and affixed to the side walls in the cargo hold of the resupply
mission via straps. It will then be launched to the ISS where it will
be removed from the cargo hold by the astronauts and stored aboard the
ISS until time for deployment.

SSTL is working with NanoRacks to
define the launch box interface with the satellite. The X50 platform
has been adapted to directly interface with the launch box via braces
all along the satellite; see the green brackets in Figure 12.
This is different from a traditional launch configuration where the
platform is hard mounted to the launcher through one interface point
(i.e. the deployment system) on the separation panel. This
configuration allows the loads to be distributed across the spacecraft
and reduces the interface loads seen by the payloads. The foam around
the box during the launch will damp out any excess vibrations as well.

Deployer Accommodation: Once
the launch box has been unpacked from the resupply vehicle, it will be
stored on board the ISS until time of deployment. When the satellite is
ready to be deployed, an astronaut will move the launch box into the
JEM ( Japanese Experiment Module) of the ISS. Here the astronaut will
position the launch box onto the small satellite deployment system,
called Kaber, within the JEM airlock.

The Kaber is a self-contained
satellite deployer system that provides an interface between the JEM
airlock slide table and the ISS SPDM (Special Purpose Dextrous
Manipulator). Kaber consists of the separation system mount plate and
the avionics/drive mechanism compartment and adapter ring. The Kaber is
stowed in the ISS for reuse for each small satellite deployment
mission. 18)

Once the platform is integrated to
the slide table, the external launch box is fully disassembled and the
support brackets are removed from the platform by the astronaut. The
platform is then slit into the JEM fairing and the airlock closed
behind it. Figure 13 shows a variant of
the X50M platform (used for RemoveDebris) within the JEM fairing for a
fit check. The platform had to be reduced in height and footprint in
order to be able to fit within this fairing. This was done by reducing
the thickness of the separation panel, reorganizing the sensors and
antennas of the platform, and also reorganizing the payload panel
accordingly.

Volume reduction: The
original platform envisioned for the RemoveDebris mission was the X50L
variant. However, as the mission evolved, a smaller platform was
required to be compliant with the launch opportunities available. The
modularity and scalability of the X50 series meant that the platform
was able to accommodate these changes late into the design cycle. The
original X50L configuration had an overall dimension of 620 x 620 x 780
mm with a total mass of 150 kg, including all payloads and propellant.
The revised baseline platform is 550 x 550 x 760 mm in overall
dimension with a mass of approximately 100 kg. Figure 14 shows the change from the X50L baseline to the X50M baseline.

Figure 14: Evolution of the RemoveDebris platform from the X50L to the X50M (image credit: SSTL)

Mass reduction: In addition
to the platform’s overall dimensions being scaled in order to
comply with the ISS launch constraints, the mass also had to be reduced
in order to be able to use the ISS deployment system. This was
accomplished by reducing the payload manifest, simplifying some of the
original demonstrations, and moving to smaller reaction wheels.

However, more mass reduction was
required. This was done by a mass lightening exercise on the platform
structure. The platform still had to keep the option open to be
launched from various other launchers as well. The launch
configurations from a traditional launch vehicle and a resupply vehicle
to the ISS have completely different requirements. The traditional hard
mounted launch configuration has higher loads that the platform
structure has to be able to survive. This meant the platform structural
panels, especially the separation panels, had to be made more robust
and was therefore more bulky. However, this was not compatible with the
ISS launch since it meant the platform was too tall and too heavy.

The mass lightening of the structure
had to be done carefully and two options were arrived upon. One was to
reduce the thickness of the separation panel (which was originally a
webbed, machined aluminum panel, designed to withstand various
launchers including a side launch configuration) to 30 mm with ISS
deployment specific mounts. The thinner panel along with the mounting
configuration in the launch box is sufficient to survive the launch
loads expected in a soft ride to the ISS. However, if a more
traditional launch emerges, it is possible to revert back to the
thicker more robust separation panel without having to dismantle the
whole spacecraft.

Changes in payload requirements:
The platform had to be able to interface with multiple payloads
mechanically, electrically, thermally, etc. The platform had to be
designed to route harnessing to various locations in order to provide
power and data connections. In addition, the platform had to have
enough switches to account for all the payloads.

This was accommodated by the new
avionics system. Since the power distribution modules in the X-Series
avionics were on individual module cards, it was possible to increase
(or decrease) the number of switches easily. In addition, the cardframe
structure was grown and split into two separate units and sized to hold
the exact number of cards that were needed for the mission (including
some spares). The CAN bus provided a distributed data interface and
allowed multiple payloads to be connected to it as well.

Removal of propulsion system:
The original mission design included a butane cold gas propulsion
system. The design of the platform allows the propulsion system, which
was designed to be a standalone unit, to be integrated at the very end
by sliding it through the opening in the bottom of the separation
panel. However, the simplification of the demonstration requirements
and reducing the operation orbit meant that a propulsion system was no
longer required. This had the added bonus of reducing the mass for the
ISS launch option and also has simplified the safety review process.

Figure 15: X50 series propulsion system (image credit: SSTL)

Drag Effects on the Launch Opportunity Selection:
In a various range of space applications, the most significant orbital
perturbations with respect to the Keplerian dynamics model are those
due to the non-sphericity of the Earth’s gravitational potential,
and especially to its first zonal coefficient J2. In our case however,
we will pay specific attention to the drag effects since, contrarily to
the gravitation perturbations, they do not affect the platform and the
different CubeSats in the same way. Indeed the relative perturbations
must be considered when dealing with formation flying or rendezvous
applications, and in Low Earth Orbit the relative drag effects can be
critical when the different orbited spacecrafts have dissymmetric
features. It has been numerically verified with the Airbus DS in-house
Mission Analysis simulation platform OSCAR that, given the
demonstration scenario and the orbited objects properties, the impact
of the Earth’s oblateness on the relative motion is insignificant
as compared to the drag contribution.

• August 2019: The RemoveDebris
mission has been the first ADR (Active Debris Removal) mission to give
in orbit demonstrations of cost effective technologies that can be used
to observe, capture and dispose of space debris. 21)

- The
craft was launched to the ISS on the 2nd of April 2018, on board a
Dragon capsule. From here the satellite was deployed via the NanoRacks
Kaber system into an orbit at 405 km altitude and has performed key
technology demonstrations including the use of a net, a harpoon, VBN
(Vision-Based Navigation) and a dragsail in a realistic space
operational environment.

Event

Date

Start of RemoveDEBRIS project (Kick off)

October 2013

PDR (Preliminary Design Review)

December 2014

Platform CDR (Critical Design Review)

March 2017

Satellite FRR (Flight Readiness Review) & AR (Acceptance Review)

December 2017

Transfer to US

December 2017

Launch

2 April 2018

Release from ISS

20 June 2018

End of LEOP & commissioning

August 2018

Net Capture Experiment

16 September 2018

Vision Based Navigation Experiment

28 October 2018

Harpoon Experiment

8 February 2019

DragSail Experiment

4 March 2019

End of Life (planned)

2020-2021

Table 5: RemoveDebris overall mission chronology

In Orbit Demonstrations

1) Net Capture Demonstration

The first demonstration to take
place was the Net capture. This demonstration required the release of
the CubeSat DSAT#1 (Debris Satellite-1), at low speed (V=5cm/s), and
this to inflate its deployable structures in order to become more
representative of the size of a large space debris.

On 16 September 2018, DSAT#1 was
deployed by the ISIS deployer as planned, and triggered by a timer its
deployables were actuated. Measurements of the velocity with which the
cubesat drifted away from the mothercarft showed a speed of
approximately 7.5m/s; slightly higher than planned but still
appropriate for the experiment. Two of the four lateral deployable
booms deployed as planned (Figure 19) and
so did the longitudinal deployable boom. The latter is visible in the
video-footage of the Net capture experiment, as recorded by the two
surveillance cameras mounted on the mothercraft to video the
experiment.

Figure 20:
Moment of the Net capture of DSAT#1, one of the satellite sails is
shown, between the lateral and longitudinal booms (image credit:
RemoveDebris Team)

Figure 20
shows the moment of the capture, at the edge of the Net, from where it
is possible to see one of the 4 sails of DSAT#1, between one of the
lateral booms and the longitudinal boom.

2) VBN (Vision-Based Navigation) Demonstration

The purpose of the VBN demonstration
was to assess the state-of-the-art of Image Processing (IP) and
navigation algorithms based on actual flight data, acquired through two
sensors: a standard high quality camera and a flash imaging LiDAR
system.

The device is shown in Figure 21,
together with the cubesat DSAT#2 that was released by the mothercraft
in order to be observed by the two cameras to then be able to
reconstruct the dynamics of the object from the “pictures”
acquired via the cameras.

Once released by the ISIPOD#2, DSAT#2 drifted away from the mothercraft at a velocity of 2 cm/s.

One of the challenges is to recognize the target independently from the background (Figure 22), and this experiment provided a wealth of real data to assess the performances and robustness of the VBN algorithms.

Lidar imagery directly delivers
information about the target distance, and this data was compared with
the measurements obtained by the GPS on board the CubeSat, which were
relayed to the mothercraft via an intersatellite link. This data and
the GPS information for the mothercraft allowed calculation of the
distance between the objects, and use of this as a reference to
establish the quality of the LiDAR measurements, which was found to be
in line with the expectations.

The harpoon demonstration was
carried out firing the Harpoon on a target representative of structural
panels on old, large satellites, which are potential targets for this
technology. The target was deployed at the end of a 1.5m long boom as
shown in Figure 24.

As the target was deployed a
significant oscillation developed on the structure with the target
oscillating around its nominal position as shown in Figure 25.

The oscillations were excited by the
inputs produced by on board equipment (AOCS), and minimizing the action
of the AOCS it was possible to stabilize the boom in order to be able
to carry out the experiment with sufficient reliability.

Figure 25:
Target deployed at the end of the boom, in green the nominal position
and in red the positions at the extreme of the scallions whose
direction is indicated by thee red arrows (image credit: RemoveDebris
Team)

On 8 February 2019, the harpoon was
fired at the target. The harpoon travelled at a speed of 19m/s and hit
the target in the center as shown in Figure 26. The target was snapped off of the end of the boom (Figure 27), due to the mechanical shock, it was retained by the harpoon which was tethered to the mothercraft.

The floating target eventually wrapped itself around the deployable boom as shown in Figure 27.

Figure 27:
Left: Target captured by the harpoon, floating in space tethered to the
mothercraft. Right: Target and tether line wrapper around the boom
(image credit: RemoveDebris Team)

4) Dragsail Demonstration

The last experiment to be performed
was the dragsail, as in any mission the deployment of this device to
deorbit the craft would be the last phase of the in-orbit operations.

This payload that was delivered for integration in the mothercraft is shown in Figure 28
and when operated, the inflatable mast extends 1m out of the enclosure
and CFRP booms deploy radially outwards to unfold the sail. Once
deployed the dragsail is very similar to that used for the CubeSat
InflateSAIL that is shown in Figure 29.

As this payload was mounted on the
back of the mothercraft in order not to interfere with the other
payloads [Net, VBN and HTA (Harpoon Target Assembly)] it was not
possible to video this experiment, as all the supervision cameras were
on the other side of the craft, to monitor the other three
demonstrations. For the Dragsail, successful demonstration would have
been indicated by a significant increase in the decay rate of the
mothercraft, by some changes in the outputs pattern of the solar panels
(as these at times would be obscured by the sails), and by an increase
in the brightness of the satellite from ground observations.

The command to deploy the sail was
given on 4 March 2019. From the ground, a small increase in the
brightness of the object (the RemoveDEBRIS mothercraft) was detected,
however, there was no significant change in the output of the solar
panels, and the decay of the altitude of the object has not accelerated
as expected (Figure 30). These factors point to a possible partial deployment of the sail.

However based on the lesson learned
from the development and MAIT of the RemoveDEBRIS’ dragsail,
improvements were made on the dragsail of the CubeSat InflateSAIL and
on two further dragsails (Figure 32) that
were used for the Spaceflight Industries’ SSO-A mission. All
three sails have deployed successfully in orbit with InflateSAIL having
already re-entered and therefore the development of the RemoveDEBRIS
dragsail had its usefulness as it paved the way for the development of
these commercial devices.

One final comment is that even
without the dragsail fully deployed, the two cubesats and the craft are
deorbiting as planned. With reference to Figure 30,
the first of the CubeSats has already been de-orbited (on 2 March
2019), the second should be de-orbited in the next few months, and the
mothercraft during its first year in orbit has already lowered its
altitude by more than 10 km, and therefore is due to completely
de-orbit, burning in the atmosphere in the next couple of years.
Considering that the platform was released in orbit at an altitude
slightly higher than planned (405 km, versus the 400 km used for the
simulations in Figure 31), the decay of the craft is consistent with what was initially predicted, reported in Figure 31, and indeed well within the current guidelines.

Figure 31:
Prediction of the decrees in orbit altitude for RemoveDEBRIS satellite,
with and without deployed dragsail (image credit: SSTL)

In summary, the
demonstrations of the Net and Harpoon target technologies have
confirmed that these are indeed viable technologies for the removal of
large space debris. The hardware will need scaling up, due to the
larger size of potential real targets, but the basic technology is
sound and the in orbit demonstrations have provided a valuable
experience to de-risk future developments.

The VBN demonstration has also been
successful, collecting a great amount of data and proving the
performance of hardware and software in the real environment.

The dragsail experiment manifested
some anomalies, however, this has paved the way for successful
commercial exploitation in the new devices that have been produced by
SSC (Surrey Space Centre).

The mission has also been successful
in getting a variety of institutions working together to tackle a
global issue, from large to small companies, universities and research
centers, sharing best practice and improving their competitiveness
(Ref. 21).

• September 18, 2018: The
RemoveDEBRIS satellite has successfully used its on-board net
technology in orbit – the first demonstration in human history of
active debris removal (ADR) technology.22)23)24)

- The
spacecraft began the experimental phase of its mission on Sunday 16
September, when it used a net to capture a deployed target simulating a
piece of space debris.

- RemoveDEBRIS was designed, built
and manufactured by a consortium of leading space companies and
research institutions led by the Surrey Space Centre at the University
of Surrey. The spacecraft is operated in orbit by engineers at SSTL (
Surrey Satellite Technology Ltd) in Guildford, UK. The project is
co-funded by the European Commission.

- Professor Guglielmo Aglietti,
Director of the Surrey Space Centre, said: “We are absolutely
delighted with the outcome of the net technology. While it might sound
like a simple idea, the complexity of using a net in space to capture a
piece of debris took many years of planning, engineering and
coordination between the Surrey Space Centre, Airbus and our partners
– but there is more work to be done. These are very exciting
times for us all.”

- Ingo Retat, Airbus RemoveDEBRIS
project head, said: “To develop this net technology to capture
space debris we spent 6 years testing in parabolic flights, in special
drop towers and also thermal vacuum chambers. Our small team of
engineers and technicians have done an amazing job moving us one step
closer to clearing up low Earth orbit.”

- In the coming months,
RemoveDEBRIS will test more ADR technologies: a vision-based navigation
system that uses cameras and LiDaR technology to analyse and observe
potential pieces of debris; the first harpoon capture technology used
in orbit; and a drag-sail that will finally bring RemoveDEBRIS into the
Earth’s atmosphere where it will be destroyed, bringing its
mission to a close.

- The US Space Surveillance Network
tracks 40,000 objects and it is estimated that there are more than
7,600 tons of ‘space junk’ in and around Earth’s
orbit - with some moving faster than a speeding bullet, approaching
speeds of 30,000 miles per hour.

- The research leading to these
results has received funding from the European Union Seventh Framework
Programme [FP7/2007-2013] under grant agreement No 607099.

• August 23, 2018: SSTL has
confirmed the successful commissioning in-orbit of the RemoveDEBRIS
spacecraft, which was deployed from the International Space Station on
20 June 2018. Spacecraft operators at SSTL have just completed a series
of tests and operations to confirm the functionality of key operating
systems such as power management, communications, propulsion, attitude
control and on-board computing, and the satellite is now ready for the
experimental phase of the mission to begin. 25)26)

- The RemoveDEBRIS satellite was
designed, built and manufactured by a consortium of leading space
companies and research institutions, led by the Surrey Space Center at
the University of Surrey and co-funded by the EC (European Commission).

- The RemoveDEBRIS mission will
perform four innovative Active Debris Removal (ADR) experiments,
beginning in October with the deployment of a net developed by Airbus
in Bremen which has been designed to capture a target CubeSat. The
mission is then scheduled to test a vision-based navigation system from
Airbus in Toulouse and CSEM (Swiss Centre for Electronics and
Microsystems) in Switzerland that uses 2D and 3D LiDaR (light detection
and ranging) technology to track a CubeSat released from the main
spacecraft. Early in 2019 a harpoon developed by Airbus in Stevenage
will be fired at 20 m/s to penetrate a target made of composite
material. Finally, the RemoveDEBRIS craft will deploy a large dragsail
to speed de-orbit into the Earth’s atmosphere.

Figure 33: Europe's RemoveDebris
orbital debris removal demo. Surrey Satellite Center video of the
European RemoveDebris active debris removal demonstration planned for
2018 after deployment of the SSTL-built satellite from the
International Space Station. The satellite was delivered to the ISS on
April 4, 2018 (video credit: AviationWeek, Published on 5April 2018)

• On June 20, 2018, NanoRacks
successfully deployed the RemoveDebris satellite from the ISS via the
Company’s commercially developed Kaber (Kaber Microsatellite
Deployer). This is the third major microsatellite deployment for
NanoRacks, and the largest satellite to ever be deployed from the
International Space Station. 27)28)29)30)

Figure 34: The RemoveDebris
satellite deployed from the International Space Station on 20 June 2018
(image credit: NASA/NanoRacks/Ricky Arnold, Ref. 30)

- RemoveDEBRIS, one of the
world’s first attempts to address the build-up of dangerous space
debris orbiting Earth, was launched to the Space Station via NanoRacks
on the 14th SpaceX Commercial Resupply Mission in early April.

Figure 35: Photo of NASA
astronaut commander Drew Feustel last week as he tackled the loading of
the RemoveDebris microsatellite into the Kibo airlock (image credit:
NASA/NanoRacks)

- The RemoveDEBRIS mission will
perform four innovative ADR experiments, beginning in October with the
deployment of a net developed by Airbus in Bremen which has been
designed to capture a target CubeSat. The mission is then scheduled to
test a vision-based navigation system from Airbus in Toulouse and CSEM
in Switzerland that uses 2D and 3D LiDaR (light detection and ranging)
technology to track a CubeSat released from the main spacecraft. Early
in 2019 a harpoon developed by Airbus in Stevenage will be fired at 20
m/s to penetrate a target made of composite material. Finally, the
RemoveDEBRIS craft will deploy a large dragsail to speed de-orbit into
the Earth’s atmosphere.

- Professor Guglielmo Aglietti,
Director of the Surrey Space Centre at the University of Surrey and
Principal Investigator for the mission, said: “After almost 5
years of development, it is exciting to finally be in a position where
we can test these extremely exciting technologies in the field. If
successful, the technologies found in RemoveDEBRIS could be included in
other missions in the very near future.” 31)

- Sir Martin Sweeting, Chief
Executive of Surrey Satellite Technology Ltd commented:
“SSTL’s expertise in designing and building low cost, small
satellite missions has been fundamental to the success of RemoveDEBRIS,
a landmark technology demonstrator for Active Debris Removal missions
that will begin a new era of space junk clearance in Earth’s
orbit.”

- The RemoveDEBRIS mission will
perform four experiments, which will be tested on two CubeSats
to-be-deployed from the larger microsatellite, acting as artificial
targets. These experiments include both the first harpoon capture in
orbit and a net that will be used on a deployed target. The team will
also test a vision-based navigation system that uses cameras and LiDaR
technology to observe CubeSats that will be released from the main
spacecraft. Finally, the RemoveDEBRIS craft will deploy a large drag
sail that will cause the orbit of the spacecraft to rapidly decay until
it destroys in the Earth’s atmosphere.

• On April 4, 2018, the SpaceX
Dragon cargo spacecraft was installed on the Harmony module (Node 2) of
the International Space Station at 9:00 a.m. EDT (14:00 GMT). 32)

Figure 36: April 4, 2018:
International Space Station Configuration. Four spaceships are docked
at the space station including the SpaceX Dragon space freighter, the
Progress 69 resupply ship and the Soyuz MS-07 and MS-08 crew ships
(image credit: NASA TV) 33)

• On April 4, 2018, the SpaceX
Dragon CRS-14 arrived at the International Space Station to deliver
more than 2630 kg of research investigations, cargo and supplies,
including NASA's Materials International Space Station Experiment
(MISSE). This is the ninth MISSE mission in the program's long history
of testing material samples in space. 34)

Figure 37: The MISSE Flight
Facility is shown here, as manufactured by Alpha Space Test and
Research Alliance. The new configuration offers multiple sides,
allowing material specimens to be exposed to the space environment from
all four orientations (ram, wake, zenith, and nadir), image credit:
Alpha Space Test & Research Alliance

• April 4, 2018: The SpaceX
Dragon capsule has arrived at the International Space Station (ISS)
after a two-day orbital chase. Astronauts aboard the ISS snagged the
uncrewed Dragon at 10:40 GMT using the orbiting lab's huge Canadarm2
robotic arm. The cargo vehicle had launched Monday afternoon (April 2)
aboard a SpaceX Falcon 9 rocket, on a contracted mission for NASA. 35)

- ISS crewmembers will soon start
unloading the 2,630 kg of cargo Dragon, which includes a number of
scientific experiments. Among them is a study designed to help optimize
plant growth in space, and an investigation into how bone marrow
produces red blood cells in a microgravity environment.

- Also aboard Dragon is an
experimental spacecraft called RemoveDebris, which will be deployed
from the ISS in the near future to test ways to clean up space junk.
Once it's flying freely, the RemoveDebris mothership will practice
hitting an onboard target with a harpoon, and it will also jettison a
small piggyback satellite and then try to bag it up with a net.

- The Dragon will remain at the ISS
until next month, when crewmembers will load it up with about 1,800 kg
of cargo from the station, SpaceX representatives have said. The
capsule will depart and maneuver its way to a splashdown in the Pacific
Ocean off Baja California, where SpaceX personnel will retrieve it by
boat.

RemoveDebris mission description CubeSats - DebrisSats (DSAT)

RemoveDebris is aimed at performing
key ADR (Active Debris Removal) demonstrations (e.g. net, harpoon,
visual based navigation (VBN), and deorbit sail) representative of an
operational mission in order to advance the TRL (Technology Readiness
Level) of key ADR technologies. The project is aimed at contributing to
global/European ADR roadmaps. A microsatellite, called RemoveSat, will
release and capture several space debris targets, called DebrisSats, in
sequence using various technologies thus demonstrating in orbit, key
ADR technologies for future missions in what promises to be the first
ADR technology mission internationally.

This mission provides unique cost
and schedule challenges along with requirements that dictate the need
for having a platform and avionics suite that can be modified late into
the development program. In addition, the mission requires a platform
that is able to host and service multiple payloads with varying
requirements. Lastly, the launch options also require the need to have
a platform design that can cope with contradicting needs.

Net CubeSat, DSAT-1:

DSAT-1 is based on a 2U CubeSat with
the following dimensions: 100 x 100 x 227 mm, where 1U (100 x 100 x 100
mm) is reserved for the avionics and the remaining space is reserved
for the inflatable structure. Figure 39 shows the CubeSat structure. Figure 39
(a) shows the outer structure when the CubeSat is undeployed. The
avionics section features an interfacing port with the deployer, a
camera that can be used to take imagery of the main platform as the
CubeSat is ejected, and the burn wire that holds the inflatable system
in place. Figure 39 (b) shows the inside
of the CubeSat where the avionics boards are both clearly visible along
with the central inflation connector system. The central connector is
designed to house the inflation system, a CGG (Cold Gas Generator), and
a solenoid valve. The role of this valve is to allow trapped air escape
during the launch phase. Prior to the deployment of the structure this
valve must be closed, to form a leak free system. The central housing
may be constructed from sections of aluminum, which are sealed using a
set of o-rings.

Figure 39
(c) shows the first deployment stage, where the burn wire is burnt and
the inflation system is released downwards by means of two high torsion
springs. Once the CGG is activated, the booms simultaneously inflate
forming the overall balloon structure. This is shown in Figure 39 (d) where the wires and balloon sheeting is not shown.

Figure 39
(e) shows the fully inflated balloon including wires and membrane which
resembles an octahedron tensegrity. To establish the shape of the
inflatable structure, several geometries were investigated. The well
proven spherical envelope was selected as a benchmark against which
other geometries were evaluated. The final design has a minimal gas
volume allowing smaller CGGs to be used and the sheeting membranes are
attached axially, rather than being bounded by the inflatable members.
This design choice reduced the sail material surface area by 25%, but
the projected area remained the same.

Harpoon CubeSat, DSAT-2:

The harpoon CubeSat, DSAT-2, can be seen in Figure 40.
The CubeSat is again a 2U, like DSAT-1 where 1U is used for avionics
and 1U is used as a harpoon capture box. As can be seen in Figure 40
(a), the CubeSat deploys four panels in the shape of a cross,
increasing the likelihood the harpoon will hit. The CubeSat has a
momentum wheel that is activated just before leaving the platform,
enabling any unwanted tumbling motion to be converted into a gyroscopic
motion. The CubeSat features a series of debris-capture materials on
each panel (a bag or net) that is designed to capture any debris
produced by the harpoon. The harpoon is designed in such a way to
contain debris from the impact face and also has an end-stop to prevent
it from passing through the impact surface; the bags collect any
potential debris from the rear of the impact panel.

Figure40
(b) shows the internal avionics in the CubeSat (discussed later). An
internally mounted camera observes the impact panels and is able to
photograph the impact. A series of LEDs light up the impact space
providing light for the camera.

In the VBN scenario, the VBN payload
on the platform will inspect the VBN CubeSat, DSAT-3, during a series
of maneuvers at a range of distances and in different light conditions
dependent on the orbit. DSAT-3 will take the same structure as DSAT-2.
The panels will deploy in the same cross shape, removing CubeSat
symmetry and enabling the VBN payload to better identify the CubeSat
attitude. DSAT-3 has similar avionics to DSAT-2, except it contains a
full AOCS system allowing full 3 degree of freedom (3-DOF) control.

Avionics: The CubeSat
avionics are primarily based on the QB50 avionics developed by the SSC
(Surrey Space Centre) and the ESL (Electronic Systems Laboratory) at
Stellenbosch University. The QB50 stack, shown in Figure 41
consists of 3 boards, the CubeComputer, CubeControl and CubeSense
boards which in conjunction provide 3-DOF attitude control to a
CubeSat.

The primary boards are shown in Figure 42.
The CubeComputer performs the CubeSat processing and contains a 32 bit
ARM Cortex-M3 including flash for in-flight reprogramming (dual
redundant), an FPGA for flow-through error correction in case of a
radiation upset on the memory and a MicroSD card for data storage. The
CubeControl controls both magnetometers and samples connected sensors.
The CubeSense contains both sun and nadir sensors.

Not all of the boards are used on each CubeSat; the boards used in each CubeSat are given in Table 6.
Note each CubeSat also requires a power board (DSAT-1 and DSAT-2 have
only a single battery, while DSAT-3 has solar panels as well). Each
CubeSat also has an ISL (Inter-Satellite Link ) that is used to
communicate with the platform. In the case of DSAT-1 this is used for
the platform to command balloon inflation and to receive imagery back
from the camera. For DSAT-2 and DSAT-3, as well as imagery data, the
ISL is also used to transmit telemetry and sensor data to the platform
and for the platform to command desired CubeSat attitudes. The CubeSats
do not have a communication link directly with Earth that consumes a
lot of power, instead all CubeSat to ground communication is via the
platform. DSAT-1 also has an additional board for control of the air
valve and the CGG system.

The 3 target CubeSats for the
RemoveDebris project are carried onboard the host satellite inside 3
dedicated CubeSat dispensers provided by ISIS (Innovative Solutions In
Space). For this particular mission ISIS is redesigning its heritage
ISIPOD CubeSat dispenser system to meet the specific mission objectives
for the project. Normally the CubeSat dispensers deploy the CubeSats
into orbit from an upper stage of a rocket and are activated within the
first hour of the launch. For RemoveDebris, the CubeSats will be
deployed from a host satellite, which causes specific integration and
accommodation challenges and in addition the CubeSats will be deployed
long after launch. This has some key implications for the dispenser
system. The dispenser now needs to withstand up to several months in
orbit before the host satellite will activate the dispensers to eject
the target CubeSats.

Moreover, the target CubeSat inside
the dispensers will also need to be stowed inside the host satellites a
long time. The dispensers will be outfitted with a special interface so
the host satellite can charge the batteries of the target CubeSats and
can perform in-space checkout routines by offering an interface to the
target CubeSat computer. Finally, a major modification specific for the
RemoveDebris dispenser compared to the normal dispensers is the
deployment velocity for the Target CubeSats. Nominally the CubeSats are
ejected with a deployment velocity of 1 to 2 m/s in order to ensure
that the CubeSats separates sufficiently fast from the launch vehicle
upper stage.

However, given the mission of
RemoveDebris, there are requirements to keep the target satellites
close to the host spacecraft so there is a minimum risk of losing sight
of the Targets. Therefore, the CubeSat dispensers are modified and
complemented with an additional low speed deployment functionality that
will allow a very low deployment velocity as low as 5 cm/s for the
deployment of the target CubeSats. Ideally, these speeds are 2 cm/s for
the harpoon demonstration and 5 cm/s for the net demonstration.

Payload/experiment complement:

Net demonstration:

A CubeSat will be released from the
main RemoveSat platform. As the simulated debris CubeSat drifts away
and reaches a specified distance from the main platform, a net will be
released to capture it. Once the net is ejected, it will expand in size
and throw weights at the edges of the net will wind the net closed
after it has captured the target. This sequence of events will be
recorded by the on board cameras from the main platform and be
downlinked to the ground.

The NETCAM (Net Capture Mechanism)
will be accommodated on top of the spacecraft bus and is provided by
Airbus DS, Bremen. After activation command from the OBC, a net will be
deployed to capture a nanosatellite released from the spacecraft
approximately 100 s before. After entanglement of the net around the
target, the net will be closed by motor driven winches, integrated to
the net deployment masses. 36)

Net Hardware: The NETCAM design is shown in Figure 43.
The NETCAM has 275 mm diameter and a height of 220 mm. The total mass
target is 6:5 kg. Usually, the net will be connected with the
spacecraft via a tether to pull the target and eventually initiate the
controlled deorbiting. However, for RemoveDebris the tether will not be
fixed on the spacecraft side to avoid two body dynamic impacts due to
the low thrust propulsion system of the platform.

A hemispherical spider type net shall be used for the RemoveDebris project (Figure 44).
The net shall have 5 m diameter. Kevlar has been chosen as material.
The total mass of the net shall be approximately 0.6 kg. A mesh size of
80 mm was selected, because this may prevent the target from escaping
even if the inflation/enlargement fails. It has been realized that not
all 196 meridian lines can really run from the circumference to the pol
(i.e. the lid of the NETCAM), because this would yield a huge bundle of
lines at the pol. For the NETCAM, only 6 lines will directly connect
the deployment masses with the lid. All other meridian lines will end
at intermediate latitude lines.

The HTA
consists of the deployable target structure and the HCS. The former is
being provided by SSC, the latter is being developed by Airbus DS
Stevenage (UK) as a mission enabling capture system for future ADR
missions. The RemoveDebris mission serves to raise the TRL of key
elements of the harpoon capture system, providing a platform to test
the technology in the space environment. The HCS is designed to
establish a hard point attachment to debris and provide a link to the
chaser via a flexible coupling. A flexible link allows for deployment
from a stand-off distance, reducing the risk to the chaser during
stabilization or towing. The HCS has several features which led to its
selection:

• Low mass and volume allowing the possibility to host multiple harpoons on a single spacecraft

• Relative simplicity leading to high reliability, low development risk and low cost

• Ability to perform comprehensive characterization of capture on ground.

The HTA structure is shown in Figure 45
where the harpoon is mounted at the top and the deployable boom at the
bottom. As the deployable boom (a coiled CPFR mono-stable boom)
extends, the target panel moves out from the structure and platform. At
1:5 m the boom stops extended and the harpoon firing experiment is
performed. The experiment is captured on the dual supervision cameras
on the platform. After the experiment, the boom is retracted to prevent
interference with the dragsail for when the dragsail boom is deployed
later in the mission.

Harpoon hardware:
The baseline harpoon concept for large debris items was developed under
internal Airbus R&D and a small scale demonstrator has been
accepted for flight test on RemoveDebris. The HCS designed for
RemoveDebris is composed of 3 main elements; Deployer, Projectile and
Tether. The flight harpoon payload is shown in Figure 46. 37)

The Deployer imparts sufficient
velocity to the projectile for penetration of the target structure.
Extensive ground characterization has established that 20m/s is
required to penetrate the targets aluminum honeycomb panels. Energy is
provided to the system by a gas generator mounted at the back of the
Deployer. Upon activating gas is released into the chamber volume,
increasing the force applied against a piston. The piston is held by a
tear pin until a set failure stress is reached, resulting in the piston
propelling the projectile out of the Deployer. To provide fault
tolerance against premature deployment, a HDR (Hold Down and Release)
mechanism is to be incorporated on the flight model.

The projectile is shown in Figure 47.
The projectile is designed to penetrate the target panel and
successfully deploy a set of barbs on the opposite side, providing the
crucial locking interface with the target. A shroud protects the barbs
during the penetration of the structure and allows the harpoon to
capture targets with misalignments of up to 45º. Free release of
the tether is a key influence on the accuracy of the HCS. Tests have
been performed to select the ideal spool arrangement and mounting
location to minimize an inaccuracy in impact location.

Harpoon and HTA testing: A
significant benefit of the harpoon is that validation of many aspects
can be performed on ground in the Airbus DS test range shown in Figure 48.
The availability of a test range allows for many of the design
challenges to be overcome and characterized on ground before use
on-orbit. The test rig has allowed many design variables to be tested;
projectile configuration, panel type, panel offset, chaser momentum.
The availability of the test rig allows the rapid prototype development
and identification of key design variables that are difficult to
identify using classical design approaches.

The opportunity for the RemoveDebris
in-flight demonstration of the HCS is the next step the development of
the technology for adoption on a future ADR mission. The mission will
demonstrate the HCS functionality in the LEO environment. RemoveDebris
will advance the HCS beyond the Airbus DS concept and breadboards and
inform the development of an integrated system design that can be
considered for future ADR missions.

VBN (Vision Based Navigation):

Airbus DS Toulouse has been strongly
involved in the design VBN systems over the last years, with particular
focus on applications such as planetary landing and orbital rendezvous,
typically in the context of MSR (Mars Sample Return) missions. Based on
this background and due to the increasing interest in ADR, solutions
for autonomous, vision-based navigation for non-cooperative rendezvous
have been investigated. Dedicated image processing (IP) and navigation
algorithms have been designed at Airbus DS and INRIA to meet this
specific case, and some of them have already been tested over synthetic
images and actual pictures of various spacecraft. As the next step, the
VBN demonstration onboard RemoveDebris will validate VBN equipment and
algorithms, through ground-based processing of actual images acquired
in flight, in conditions fully representative of ADR. The VBN
demonstration will thus fulfil the following objectives:

• Demonstrate state-of-the-art
image processing and navigation algorithms based on actual flight data,
acquired through two different but complementary sensors: a standard
camera, and a flash imaging LiDAR.

• Provide an on-board processing function in order to support navigation.

VBN hardware: Images will be
captured from two main optical sensors: a conventional 2D camera
(passive imager) and an innovative flash imaging LiDAR (active imager),
developed by CSEM (Swiss Centre for Electronics and Microsystems). It
will be a scaled-down version of a 3D imaging device currently
developed and tested in the frame of ‘Fosternav’ FP7
project for the EC (European Commission) focusing on landing and
rendezvous applications. This architecture has the particularity of
providing ranging capability by measuring the phase difference of two
signals. It will be the first time in Europe that a device based on
flash imaging LiDAR technology - considered to be a key enabling
technology by the space community for the future success of exploration
missions with landing, rendezvous and rover navigation phases -will be
used for debris tracking and capture control. Such an experiment will
allow Europe to master state of the art technologies in the field of 3D
vision sensors for GNC systems. The hardware is shown in Figure 49.

Demonstration trajectory: In
a first step, 3D and 2D images will be captured from the start of the
operational phase, i.e. when DSAT-1 is released for preliminary checks,
monitoring purposes, as well as a first collection of data covering the
net experiment. In a second step, the VBN demonstration per se will
start, and will consist in capturing images of DSAT-3 from various
distances and over large duration in order to make sure that the widest
range of visual configurations (in terms of distance to target,
relative attitude, light conditions, background) is reached. This will
make the experiment as much demonstrative as possible, while meeting
the classical duration and cost constraints of a low-cost demonstration
mission.

Starting
from DSAT-3 ejection, several types of rendezvous maneuvers are
possible, and a reference scenario made up of hops trajectories based
on radial (R-bar) or velocity (V-bar) burns is being defined. These
maneuvers are all standard and representative of future debris removal
proximity operations, such as final rendezvous, inspection and capture.
A possible trajectory combining these different maneuvers so as to
maximize the range of visual configurations between RemoveSat and
DSAT-3 while minimizing propellant consumption, is illustrated in
Figure 50.

After DSAT-3 deploys a dragsail that
will hasten its orbital decay, the 3D and 2D cameras will continue to
collect imagery as long as LOS (Line-Of-Sight) is maintained. Image
data will be downloaded during ground contact windows.

On-ground processing: All the
data acquired during the VBN experiment will be processed on the ground
with innovative IP algorithms (e.g. 2D/3D and 3D/3D matching
techniques) and specifically tuned navigation algorithms based on an
EKF (Extended Kalman Filter) able to fuse data from different sensors
(e.g. camera images and attitude sensing data).

Differential GPS and onboard
attitude estimation software will also provide ‘ground
truth’ data against which the navigation algorithms will be
compared for validation and performance assessment. Post-processing
activities will allow demonstration of performances of innovative 2D
camera based navigation and 3D camera based navigation, allowing not
only estimation of relative position and velocity but also relative
attitude, one of the key drivers of successful capture of an
uncooperative target.

Supervision Cameras:

The
RemoveSat platform will house two supervision cameras: one dedicated to
the net demonstration with a 65º x 54 º FOV (Field of View)
and one dedicated to the harpoon demonstration with a 17º x
14º FOV.

The supervision cameras are based on SSTL’s heritage system, shown in Figure 51,
flown on the TDS-1 (Technology Demonstration Satellite-1) launched in
July, 2014. This camera system uses COTS technology combining a color
CMOS camera with a high performance machine vision lens capable of
delivering video. Both camera and lens are stripped down and all
unsuitable components removed before being ruggedized during reassembly
to survive the vibration and shock loads experienced during launch as
well as making it suitable for the space environment. The camera system
will be optimized to give a depth of field capable of meeting the
performance requirements for the two demonstrations. Customized
mounting brackets will be used to point the camera in the required
direction for the demonstrations. The cameras will use a CameraLink
interface to the PIU. They will acquire 8 bit images that are 1280 x
1024 pixels in size at 10 frames/s. Figure 20 shows an image taken of
the APM (Antenna Pointing Mechanism) on TDS-1 just after launch with
Earth in the background.

Figure 52: Image from camera on TDS-1 of APM with Earth in the background (image credit: RemoveDebris consortium)

Dragsail:

The RemoveDebris platform will have a SSC (Surrey Space Centre) dragsail payload. The dragsail concept can be seen in Figure 53.
The dragsail consists of 2 parts: a deployer which extends the sail
away from the platform (preventing the sail from hitting any
overhanging platform hardware e.g. antennas), and a extension mechanism
which uses a motor to unfurl carbon fiber booms that hold the sail
membrane. Figure 54 shows both of these
mechanisms. The deployer is a mechanical two stage ejector that uses a
series of springs to eject the system. The extension mechanism consists
of four booms rolled into a central distributor that allows controlled
unfurling of the sail.

Figure 53: Schematic of the dragsail concept (image credit: SSC)

Figure 54: Photo of the dragsail payload (image credit: SSC)

Figure 55
shows an external and internal view of the dragsail. The deployer is an
inflatable mechanism that deploys to a length of 1 m and self-hardens.
The extension mechanism consists of four booms rolled into a central
distributor that allows controlled unfurling of the sail.

Figure 55: Illustration of the dragsail payload (image credit: SSC)

ADR regulatory issues:

The RemoveDebris mission intends to
fully comply with all relevant national and international space laws.
In particular, it is of prime importance that all space elements
released into orbit deorbit within 25 years as demonstrated in the
deorbit analysis section. The net and harpoon are also being designed
to be trackable from space for the very worst case they miss their
targets. Both of these guarantee even if these items miss they will
deorbit within 25 years and can be fully tracked during this period. It
is also important to not contribute to the production of further debris
in space.

For
example special provisions have been placed on the addition of debris
containment bags around DSAT-2, which help ensure that when the harpoon
hits the CubeSat target any debris which might be produced does not
escape, even if extensive ground tests show that debris is not created
from the kinetic impact of the harpoon. Finally, CubeSats are used here
as artificial debris targets; this avoids any legal issues with
targeting, capturing or deorbiting debris that is legally owned by
other entities.

The RemoveDebris consortium aims to
work with the EU, UKSA (UK Space Agency), ESA, CNES and other agencies
/entities to provide the latest project achievements, incorporate their
feedback, communicate and interface with them on all necessary
regulatory procedures required for the RemoveDebris mission.

Ground segment:

Operations for the RemoveDebris
mission will be carried out from SSTL’s MOC (Mission Operations
Centre) in Guildford as shown in Figure56.
SSTL’s standard operations procedures will be used, which are of
course compatible with the SSTL designed platform operational
requirements and characteristics.

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates (herb.kramer@gmx.net).